Oil strainers for removing impurities and other substances contained in the oil have been disposed in the oil pan of an engine or a transmission, etc., of a vehicle (see, e.g., Patent Document 1). The oil strainer of Patent Document 1 has a filter and an elongated casing in which the filter is housed. The filter is comprised of a mesh portion in a plate-like shape, and a frame that surrounds the mesh portion. The mesh portion is reinforced by the frame and is supported on the casing. The casing is provided with an oil inlet hole and an oil outlet hole near the ends in its longitudinal direction. The oil which has flowed into the casing from the oil inlet hole is strained through the filter, and thereafter flows out from the oil outlet hole.